This invention relates generally to microwave devices and more particularly to microstrip couplers.
As is known in the art, microstrip couplers generally include a pair of strip conductors formed on one surface of a dielectric support having a conductive ground plane formed on the opposite surface of such support, such strip conductors being disposed adjacent one another for an electrical length .lambda..sub.o /4 where .lambda..sub.o is the midband operating wavelength of the coupler. The coupling factor of such coupler is related to the width of the gap separating the adjacent strip conductors and the impedances of the microstrip transmission lines formed with such strip conductors. The strip conductors are typically formed using photolithographic techniques. When such techniques are used to form such couplers, it is generally difficult to control the width of the gap between the strip conductors. For example, when such photolithographic process is used to form strip conductors of copper clad on a relatively soft dielectric support, the smallest controllable width of such gap is in the order of 1.5 mils. Therefore, using such conventional photolithographic techniques, the minimum coupling obtainable is approximately 8-10 db.
In order to achieve tighter coupling, say 3-6 db, more complex designs, such as interdigitated couplers and tandem couplers, are used in microstrip transmission line mediums or overlapping strip conductors in a stripline medium. The interdigitated coupler and tandem couplers are relatively large, require extremely tight tolerances on the photolithographic etching process, and must be fabricated on a dielectric support having a relatively high dielectric constant with a high quality surface finish. With regard to overlapping couplers formed in a stripline medium, many multifunction microwave circuits use a microstrip medium for switch, attenuator and phase shifter functions, and when a coupler having a coupling value of less than 10 db is required, the transition to a stripline medium is relatively costly and mechanically unreliable over a wide temperature range.